This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Prenatal steroids are routinely used in the care of women who show signs of or have risk factors for premature birth. These steroids stimulate lung development in premature infants, leading to decreased mortality rates. Our work has shown that exposure to a multi-course injection (2 injections/week at 65%, 75%, and 85% gestation) of betamethasone leads to increased oxidative capacities of the slow- and fast-twitch fibers of the fetal guinea pig (Cavia porcellus) scalenus, an accessory inspiratory muscle. Therefore, the purpose of this study is to examine the effects of prenatal steroid exposure, both single (2 injections at 70% gestation) and multi-course, on the metabolic and functional properties of guinea pig breathing muscles. We propose to test the hypotheses that prenatal steroids lead to changes in the (1) oxidative capacities and (2) fatigue resistance of five guinea pig ventilatory muscles. Pregnant guinea pigs will be injected intramuscularly with betamethasone (0.5 mg/kg of body weight) or sterile water according to the presented single and multi-course injection schedules. Fetuses will be delivered 24 hours after the final injection, and breathing muscle samples will be collected. Histo- and immunocytochemical methods will be used to compare the (1) NADH-tetrazolium reductase activities in slow- and fast-twitch fibers and (2) capillary densities (number of capillaries per fiber cross-sectional area) of the ventilatory muscles in steroid-treated and untreated fetuses to determine whether prenatal steroid exposure leads to increases in both of these measures of oxidative capacity. Whole-muscle citrate synthase (CS) activities will also be measured biochemically to assess whether prenatal steroids increase CS activity levels in treated muscles. Finally, the fatigue resistance of these muscles will be determined. If the treated muscles express more oxidative enzymes, greater capillary densities, and fatigue resistance, these muscles will be better prepared to support the ventilatory needs of premature infants. Public Health Relevance: Prenatal steroids, routinely given to women who show signs of or have risk factors for giving birth prematurely, are known to accelerate fetal lung development, allowing newborns to breathe more competently if they are born prematurely. Whereas corticosteroids have detrimental effects in mature skeletal muscle, prenatal steroids may accelerate the acquisition of the metabolic machinery necessary to sustain ventilation in these premature infants, better preparing them for life outside the womb.